1. Field of the Invention
The present invention relates to a displacement detecting device, a scale calibrating method and a scale calibrating program applied to a linear encoder, a rotary encoder, etc.
2. Description of the Related Art
Generally, measurement error of a displacement measuring device such as an encoder is evaluated before shipment. A highly accurate displacement sensor such as a laser interferometer is used for a reference for error evaluation. The thus obtained error data are shipped in the form of a pre-shipment inspection table together with the encoder so as to be used as important data for warranting performance of the encoder.
However, the scale of the encoder may be distorted according to the material and length of the scale and the fixing method when the scale of the encoder is attached to an application such as a machine tool, a measuring device, etc. In some cases, non-negligible level measurement error in regard to a required specification may be caused by the generated distortion of the scale so that reliability of error data evaluated in advance will be spoiled.
As a method for solving this problem, it is thought of that a reference displacement sensor is set up in a user's application to apply on-machine calibration to the measurement error of the encoder. It is however undesirable that a burden is imposed on the user in consideration of the labor for setting up the displacement sensor and the price of the highly accurate displacement sensor.
On the other hand, for example, methods for self-calibration measurement error on graduations of a scale (JP-A-2008-224578 and “Satoshi Kiyono, “Intelligent Precision Measurement”, The Japan Society for Precision Engineering, 2009, Vol. 75, No. 1, pp. 89-90”) are known in this type displacement detecting device. Use of these self-calibration methods permits measurement error of an encoder to be calibrated without any highly accurate displacement sensor set up in an application.
However, when configuration is made in such a manner that a plurality of sensors are arranged at intervals of predetermined distance as disclosed in JP-A-2008-224578 and “Satoshi Kiyono, “Intelligent Precision Measurement”, The Japan Society for Precision Engineering, 2009, Vol. 75, No. 1, pp. 89-90”, the sampling interval of measurement error becomes equal to the pitch of arrangement of the sensors. For this reason, measurement error having a period not longer than twice as long as the arrangement pitch cannot be restored correctly, so that the frequency of measurement error allowed to be calibrated is limited.
Although it may be thought of that the pitch of arrangement of the sensors is narrowed to solve this problem, such a minimum distance that the sensors do not interfere with one another physically is required as the arrangement pitch. For this reason, narrowing the pitch of arrangement of the sensors is limited.
Moreover, use of a highly accurate displacement sensor such as a laser interferometer or preparation of a reference sensor or the like is not desirable because configuration becomes uselessly expensive. When measurement error of a non-negligible level is caused by distortion of the scale at the time of mounting or the like, it may be necessary to set up the reference displacement sensor again and a lot of cost and labor is still required.